1. Field of the Invention
The present invention relates to an image-forming lens for use in a document-reading unit in a facsimile or digital complex machine and various image scanners, an image reader, and an image-forming device.
2. Description of the Related Art
An image reader for use in a facsimile or digital complex machine is configured to reduce image information to be read by an image-forming lens, and to convert the image information into image signals by imaging the image information on an imaging element such as a CCD. A three-line CCD in which a plurality of light-receiving elements having red, green and blue filters is arranged in three lines in one chip, for example, is used for reading color document information in full color. The document image is resolved into three primary colors by imaging the document image on the light-receiving surface of the three-line CCD, and the color image information is converted into signals by such an optical system.
FIG. 1 illustrates one example of an optical system for converting the color information into signals. A document 2 having an image to be read is flatly placed on a contact glass 1 such as a platen, and an illumination unit 3 is disposed below the contact glass 1. A slit portion extending in the front direction of the document 2 is illuminated by using the illumination unit 3 including a long light such as a xenon lamp or halogen lamp and a reflector.
The reflection light from the illuminated part of the image in the document 2 is sequentially reflected by folding mirrors M1, M2, M3, M4, M5, and a reduced image of the document image is formed on the imaging surface of a line sensor 6 as a photoelectric conversion element.
The illumination unit 3, folding mirrors M1, M2, M3, M4, M5, image-forming lens 5, and photoelectric conversion element 6 are integrally held as a reading optical unit 7. The reading optical unit 7 travels in the arrow direction (right direction in FIG. 1) by a not-shown driver, and reads the information of the entire document by displacing the reading optical unit 7 to a position illustrated by the dashed line.
The line sensor 6 as an imaging portion includes photoelectric conversion elements 6A, 6B, 6C having red (R), green (G) and blue (B) filters as a color-resolving unit, and converts the document image into image signals by illuminating and scanning the three-line CCD (three-line sensor) in which three lines are arranged in one chip. The document 2 is thereby read, and the color image of the document 2 is resolved into three primary colors of red (R), green (G) and blue (B).
The image reader is a device which reads an image in full color, and includes a color-resolving unit. The color-resolving unit is provided in an imaging optical path of the image-forming lens 5, and has red (R), green (G) and blue (B) filters 6A, 6B, 6C provided in the three-line CCD 6.
Another method different from the above color-resolving method includes a method of resolving a color into red (R), green (G) and blue (B) by selectively inserting a color-resolving prism and a filter between the image-forming lens and the line sensor. Moreover, such a method includes a method of illuminating a document by sequentially illuminating red (R), green (G) and blue (B) light sources. Document information can be read as a monochrome without providing the color-resolving element in the imaging optical path.
The image reader includes the five folding mirrors, but the number of folding minors can be increased or decreased according to the size of the reading optical unit 7 or the like.
A high contrast in a high spatial frequency region is generally required in an imaging surface of an image-forming lens for use in such an optical system. An opening efficiency close to 100% is requested until an angle of view peripheral part. It is also necessary to conform the imaging position of the respective colors of red (R), green (G) and blue (B) on the light-receiving surface, and to preferably correct chromatic aberration of respective colors, in order to preferably read a color document. It is also necessary to reduce a conjugation length, and have a wide-angle lens in order to achieve the downsizing of the optical system.
As such a wide-angle reading lens, Patent Document 1 (JP3862466B) and Patent Document 2 (JP2002-244033A) describe an imaging reading lens. The image-forming lens described in Patent Documents 1, 2 includes a configuration having the relatively small number of lenses such as a five-group and five-lens configuration, and a half-angle of view of about 30°. Such an image-forming lens uses a so-called anamorphic lens having a refractive power different in the orthogonal direction. Therefore, such an image-forming lens includes many processing problems, and the costs of the image-forming lens are also increased.
As an image-forming lens using an axisymmetric aspheric surface, Patent Documents 3 (JP3939908B) and Patent Document 4 (JP4496231B) describe an image-forming lens. However, the image-forming lens described in Patent Documents 3, 4 includes a narrow half-angle of view of about 21.5°, and a long conjugation length of an optical system. Therefore, downsizing cannot be achieved.